Wireless Energy Transfer
Inductive coupling is used in a number of wireless energy transfer applications such as charging a cordless electronic toothbrush or hybrid vehicle batteries. In coupled inductors, such as transformers, a source, e.g., primary coil, generates energy as an electromagnetic field, and a sink, e.g., a secondary coil, subtends that field such that the energy passing through the sink is optimized, e.g., is as similar as possible to the energy of the source. To optimize the energy, a distance between the source and the sink should be as small as possible, because over greater distances the induction method is highly ineffective.
Resonant Coupling System
In resonant coupling, two resonant electromagnetic objects, i.e., the source and the sink, interact with each other under resonance conditions. The resonant coupling transfers energy from the source to the sink over a mid-range distance, e.g., a few times of the resonant frequency wavelength.
FIG. 1 shows a conventional resonant coupling system 100 for transferring energy from a resonant source 110 to a resonant sink 120. The general principle of operation of the system 100 is similar to inductive coupling. A driver 140 inputs the energy into the resonant source to form an oscillating electromagnetic field 115. The excited electromagnetic field attenuates at a rate with respect to the excitation signal frequency at the driver or self resonant frequency of the source and the sink for a resonant system. However, if the resonant sink absorbs more energy than is lost during each cycle, then most of the energy is transferred to the sink. Operating the resonant source and the resonant sink at the same resonant frequency ensures that the resonant sink has a low impedance at that frequency, and that the energy is optimally absorbed. Example resonant coupling systems are disclosed in U.S. patent applications 2008/0278264 and 2007/0222542, incorporated herein by reference.
The energy is transferred, over a distance D, between resonant objects, e.g., the resonant source having a size L1 and the resonant sink having a size L2. The driver connects a power provider to the source, and the resonant sink is connected to a power consuming device, e.g., a resistive load 150. Energy is supplied by the driver to the resonant source, transferred wirelessly and non-radiatively from the resonant source to the resonant sink, and consumed by the load. The wireless non-radiative energy transfer is performed using the field 115, e.g., the electromagnetic field or an acoustic field of the resonant system. For simplicity of this specification, the field 115 is an electromagnetic field. During the coupling of the resonant objects, evanescent waves 130 are propagated between the resonant source and the resonant sink.
Coupling Enhancement
According to coupling mode theory, strength of the coupling is represented by a coupling coefficient k. The coupling enhancement is denoted by an increase of an absolute value of the coupling coefficient k. Based on the coupling mode theory, the resonant frequency of the resonant coupling system is partitioned into multiple frequencies. For example, in two objects resonance compiling systems, there are even and odd mode resonant frequencies, due to the coupling effect. The coupling coefficient of two objects resonant system formed by two identical resonant structures is calculated by a partitioning of the even and odd modes according toκ=π|feven−fodd|  (1)
It is a challenge to optimize the coupling. For example, to optimize the coupling, resonant objects with a high quality factor are selected. However, that solution is based on a design of a particular resonant system, which may not be applicable to other resonant systems.
During the coupling of the resonant objects, the evanescent wave 130 is propagated between the resonant source and the resonant sink, which confine the energy within the system. A range of the energy transfer, i.e., the distance D, is limited to about 3 to 8 times the characteristic size of the resonant source. For example, for a 30 cm loop, the range is approximately between 90 cm to 2.5 m. In addition, the coupling and hence the efficiency decrease with the distance between the source and the sink.
Accordingly, it is desired increase the range of the energy transfer between the source and the sink.